In radio systems, such as one for cellular phones, for example, radio services must cover communication areas in a planar fashion. Further, the range of radio transmission is limited and all of the service areas cannot be covered by a single base station (“access point”; to be hereafter referred to as “access point AP”). Accordingly, a plurality of access points AP are provided so that communications can continue despite the movement of terminal stations.
Specifically, a system called “cell configuration” is employed, as shown in FIG. 9. Referring to FIG. 9, each hexagon represents a cell 32 at the center of which an access point AP 31 is located. In each cell 32, a terminal MT 33 is controlled by the access point AP 31 and communications are carried out between the access point AP 3 and the terminal MT 33.
In this configuration, in order to allow services, such as telephone services, to continue when the terminal MT moves, the individual cells 32 must be arranged at least in an adjoining manner or in such a manner that they have partly overlapping areas therebetween. However, the adjoining cells must communicate among themselves at different frequencies so as to avoid their radio waves from entering each other and interfering with each other. When communications are carried out at the same frequency, if the communicating cells are apart from each other with a distance corresponding to several cells, for example, the intensity of the interfering waves are sufficiently reduced that no problems are caused. This type of system is used in PDC (Personal Digital Cellular), for example, which is a current cellular phone system.
However, since in this type of system the frequencies that can be actually used in a single cell are only a fraction of the frequencies that are allocated to the entire system, the volume of lines that can be accommodated in a single cell is limited.
There has been a proposal to configure cells at the same frequency using TDMA (Time Division Multiple Access) technology. With reference to FIG. 10, the method of utilizing frequency and time in this technology will be described. FIG. 10(a) shows the relationship between time and frequency in the FDMA (Frequency Division Multiple Access) method. FIG. 10(b) shows the relationship between time and frequency in the TDMA method.
As shown in FIG. 10(a), in the FDMA method, each user is allocated a different frequency, so that the same frequency is occupied by a particular user on the time axis during communication. Further, since there are a plurality of users in each single cell, a plurality of frequency channels are allocated to each cell. For example, there are users in cell 1 that conduct communications using frequencies f1 to f4. In cell 2, there are users who communicate using frequencies f5 to f8.
As shown in FIG. 10(b), the TDMA method employs one frequency (band) that is divided into small slots on the time axis, and the users conduct communications using their assigned slots. When carrying out communications, a slot must be allocated to each user repeatedly. For this purpose, a control is performed whereby, for example, a user is allocated in each period where a repetition period is taken as one cycle.
With reference to FIG. 11, the method of using time slots in a case where a second access point AP2 exists in a cell adjacent to a first access point AP1.
It is now assumed that the two access points AP1 and AP2 operate as communication systems of the TDMA method and that they have the same number of time slots (repetition periods) and the same time slot times that are synchronized between the first and second access points AP1 and AP2. The system shown in FIG. 11 have eight time slots TS1 to TS8, for example.
As shown in FIG. 11, the first access point AP1 and a terminal (not shown) communicate with each other using a second time slot TS2. In this case, therefore, the time slots TS1 and TS3 to TS8 are vacant.
The use of the second time slot TS2 for communications between the second access point AP2 and the terminal would result in a large radio interference due to the communications going on between the first access point AP1 and the terminal. Therefore, the communications between the second access point AP2 and the terminal are carried out using a time slot selected from the remaining seven time slots other than the second time slot TS2.
By thus using the same frequency and dividing the time domain into a plurality of time slots, the frequency can be shared by different access points AP for communications among one another.
In the conventional frequency division method, it is difficult to freely change the frequency width due to the limitations of analog circuits, such as filters. On the other hand, there are no circuitry limitations in the TDMA method since this method divides on the time axis. It is also possible for a single terminal to use not just one time slot but two or three time slots. In this way, the communication volume can be increased by two or three folds, allowing bands to be freely controlled in multimedia communications, for example.
Thus, the TDMA method is also advantageous for communications in which the transmission volume varies constantly, such as packet data communications.
In a communication system based on the TDMA method in which the cell configuration is adopted as described above, the vacancy state of time slots as minimum units has a large influence on the number of terminals that can be accommodated. In the case where cells are assembled at the same frequency, the number of terminals that can be accommodated in each cell is determined by the number of time slots, the interference distance, and the cell arrangement.
In radio communications, the received power (C) versus noise power (N) ratio (C/N), or the received power (C) versus interference power (I) ratio (C/I) that are required for communications are determined by the performance of the system or the terminals. If these conditions are not met, problems arise in communications. The expression (C/(N+I)), in which both of the above-mentioned influences are taken into consideration, might also be used as the indication of whether or not communications are possible.
Referring to FIG. 11, a time slot that can be used in a predetermined time slot TS is that slot on which the influence of interference due to other radio stations is not more than the above expression C/(N+I). For example, a time slot in which an interference power exceeding an allowable power exists cannot be additionally allocated to an access point AP.
Consequently, a terminal that tries to connect to an access point AP has to measure the received power or the C/(N+I) ratio of every time slot and then notify the access point AP of the obtained information so that the access point AP can then allocate to the terminal a time slot that has no influence of interference and that is a vacant slot in the relevant cell.
Further, when a packet communication is carried out in a TDMA system, a particular terminal does not occupy a time slot constantly, but rather vacant slots change all the time. Therefore, the terminal must monitor the occupation state of every time slot continuously. This means that the terminal is in a receiving state all the time, resulting in increasing power consumption and other problems.
Moreover, as the number of terminals increases and the influence of the radio waves used by adjacent cells increases, there could even be cases where none of the time slots are available, putting a particular terminal in a busy state. In such cases, communications with the particular terminal cannot be conducted until its communication with another terminal ends, the interfering station ends communications, or the radio environment changes as the station attempting connection caused by movement of the station, for example.
There is another problem relating to handoff. When a communicating terminal moves across cells in a service area divided into cells, the access points AP with which the terminal communicates are switched in an operation (“handoff”).
There are two types of handoff. One is called a hard handoff whereby the connection of a line between the terminal and an access point AP toward which the terminal is approaching is established after the connection of a line with the access point AP from which the terminal is moving is cut. The other is called a soft handoff whereby the connection of a line between the terminal MT and an approaching access point AP is established before disconnecting the connection of a line with the original access point AP, such that the weight of connection is gradually shifted from that with the original access point AP to the approaching access point AP.
Since the TDMA method employs the same frequency for communications, the method characteristically realizes a soft handoff procedure that allows the terminal MT to communicate with the original and approaching access points AP simultaneously.
In the case of soft handoff, if the time slot that has been used by the original access point AP is vacant in the approaching access point AP, the same time slot is allocated for soft handoff. The terminal MT is thus allowed to carry out communications with the two access points AP at the same frequency and in the same time slot by emitting only one type of radio wave, thus realizing the soft handoff on the part of the system.
FIG. 12 shows the manner in which the time slots are utilized in the above-described case. It is assumed that the two access points AP1 and AP2 are operated as TDMA radio communication systems and that they have the same number of time slots (repetition periods) and the same time slot times that are synchronized. The system shown in FIG. 12 involves eight time slots, for example.
A first access point AP1 is communicating with a first terminal that exists in a cell of its own station, using a second time slot TS2. A second access point AP2 is communicating with a second terminal in a cell of its own station which is different from the first terminal, using a sixth time slot TS6.
It is now assumed that a soft handoff procedure is to be performed and that a third terminal, which is about to initiate soft handoff, is communicating using a fourth time slot. In this example, the soft handoff procedure can be performed because the second access point AP2 is not using the fourth time slot TS4.
However, if a third access point AP (not shown) that is located adjacent to or has a strong influence on the second access point AP2 is using the fourth time slot TS4 such that, even though the second access point AP2 is not using the fourth time slot TS4, there is a strong interference, the second access point AP2 may in some cases not be able to use the fourth time slot TS4 for communications. In such cases, the soft handoff procedure cannot be performed.
As explained above, while the TDMA system has the advantage of soft handoff, as the density of terminals increases, it becomes increasingly difficult to select a vacant time slot that is common to the two access points AP1 and AP2 and that has a reduced interference.
It is therefore an object of the invention to provide a radio communication system that does not require the monitoring of each time slot by the terminals. It is another object of the invention to provide a technology that can eliminate situations where a terminal cannot conduct communications because none of the time slots are available or situations where soft handoff cannot be performed.